Control and protection of a doubly-fed induction generator system

ABSTRACT

A control system for a double-fed induction generator (DFIG) comprising a rotor ( 1 ) having rotor windings and a stator ( 2 ) having stator windings connectable to a grid for electric power distribution. The control system comprises a converter ( 17 - 0, 171 ), having a clamping unit comprising at least one passive voltage-dependent resistor element ( 291, 292, 293, 294 ) for providing a clamping voltage over the rotor windings when the clamping unit is triggered. The invention also relates to a double-fed induction generator (DIFIG) system and to a method for protecting the converter in a power generation system.

TECHNICAL FIELD

The invention relates to the control of a Double-fed Induction Generator(DFIG), especially for use in wind-power generation.

STATE OF THE ART

One of the basic problems involved with the generation of electricenergy using wind-power is the fact that the turbine speed should beable to vary according to the wind speed, in order to improve energyefficiency and reduce the mechanical loads on the wind-turbine. However,in spite of the variations in turbine speed, the output power from thewind-power generator should be kept at a constant frequency,corresponding to the frequency of the electric power distributionnetwork or grid to which the generator is coupled. That is, VariableSpeed Constant Frequency (VSCF) is desired. In wind-power generation, inorder to achieve VSCF operation, Double-fed Induction Generators (DFIGs)have been used; systems involving DFIGs are disclosed in, for example:

Pena, R. S., et al., “Vector Control of a Variable Speed Doubly-FedInduction Machine for Wind Generation Systems”, EPE Journal, Vol. 6, no3-4, December 1996, pp. 60-67

Weiss, H., “Rotor Circuit GTO Converter for Slip Ring InductionMachines”, ENE-97 (Trondheim), pp. 2717-2728

JP-A-07-067393

JP-A-07-194196

A conventional DFIG system is shown in FIG. 1. A rotor 1 of a generatorcomprising an electric multiphase (in this case, 3-phase) asynchronousmachine is connected, through a gear-box 4, to a shaft 5 driven by theblades 3 of a wind-turbine. The windings of the stator 2 of thegenerator are connected, through a switch 6, to output lines 100connected to a transformer 101 by means of which the output lines areconnected to the electric power distribution network or grid 102,normally a medium voltage (10 kV-40 kV) grid. The voltage on the outputlines from the stator is normally in the order of 690 V (considered tobe the normal operation voltage level of the stator).

The system further includes a converter 7 having a rotor-side inverteror rotor-inverter (71, 72, 73) connected to the windings of the rotorthrough control lines 8, each control line including an inductor 9. Theconverter 7 further comprises a grid-side inverter or grid-inverter (74,75, 76) connected to the above-mentioned output lines 100 (and, thus, tothe grid) through grid-inverter connection lines 103, coupled to atransformer 104 (typically, for transforming from a level of 480 V onthe converter side to 690 V on the output line side). The transformer isconnected to the output lines 100 through a switch arrangementcomprising two switches arranged in parallel: a main switch 105 arrangeddirectly between the output lines 100 and the transformer 104, and acharge switch 106 connected in series with a charge resistor 107. Thatis, the grid-inverter is connected to the grid and to the statorwindings, through the transformer 104.

The two inverters are, basically, symmetrical; each one of saidrotor-inverter and grid-inverter comprises three half-bridges (71, 72,73; 74, 75, 76) connected in parallel, one half-bridge for each phase ofthe generator and grid, respectively.

The rotor-inverter (71, 72, 73) is fed by a DC-link 77. Thegrid-inverter (74, 75, 76) controls the voltage over the DC-link 77.

Each half-bridge (71, 72, 73; 74, 75, 76) is made up of two identicalunits connected in series, each unit comprising an IGBT (Insulated GateBipolar Transistor) 78 connected in parallel with a free-wheel diode 79.

Between the two units making up each half-bridge, the half-bridge isconnected to:

the respective control line 8 (for half-bridges 71-73 of therotor-inverter); or

the respective grid-converter connection line 103 (for half-bridges74-76 of the grid-inverter).

The operation of the IGBTs 78 of the inverters (71-76) is controlled bya control module 80, arranged to receive a plurality of input signalscorresponding to the values of several parameters of the system,including:

I_(G): current in the output lines 100 at the point of connection to thetransformer 101 for connection to the grid (considered to be the“current-supplied to the grid”);

U_(G): voltage in the output lines 100 (considered to be the “voltagesupplied to-the grid”);

I_(S): current in the output lines 100 at the end connected to thestator, between the switch 6 and the connection to the branch forsupplying the converter (via the switches 105 and 106 and transformer104) (considered to be the “stator current”);

U_(S): stator voltage, measured at the stator windings (between thestator 2 anti the switch 6);

I_(L): current in the control lines 8 connecting the rotor windings andthe rotor inverter to each other; and

the rotational velocity of the rotor, measured by an encoder 85.

With these inputs, the control module 80 can control the PWM (PulseWidth Modulation) of the two inverters.

The control module 80 receives a power reference signal (PRS) comingfrom the main wind-turbine controller (not shown in FIG. 1), which isarranged to receive information such as the actual power-supplied by thegenerator, the positions of the blades, wind-speed, etc. The mainwind-turbine controller is responsible for the total operation of thewind-turbine and controls a plurality of sub-controllers, including theconverter 7.

In the converter 7, the power reference signal is compared with themeasured power (based on the measured values of I_(G) and U_(G)) and theoutput of a power regulation loop of the control module 80 controls thePWM of the rotor-inverter. The DC-link is controlled by thegrid-inverter. The DC-link voltage is constant when the converteroperates under normal conditions. In the circuit of the present example(FIG. 1), the DC-link voltage can, under normal conditions, be around800 V_(DC).

Basically, the converter 7 operates as follows:

To initiate operation of the converter, the charge switch 106 is closed.Then the DC-link 77 will be charged over the charge resistor 107 and thefree-wheel diodes 79 of the grid-inverter. The voltage over the DC-linkis measured by the control module 80. When the voltage over the DC-linkreaches a pre-determined level, the main switch 105 is closed and thecharge switch 106 is opened.

After the main switch 105 is closed, the grid-inverter is started andthe DC-link voltage will be controlled by the grid inverter, so as tokeep the voltage over the DC-link at a rated value (in this example,around 800 V_(DC)). The grid-inverter can supply the grid with power(like a generator) or it can take power from the grid (like a motor).The grid-inverter operates in accordance with the voltage over theDC-link: if this voltage tends to increase (due to input from therotor-inverter), the grid-inverter supplies power to the grid; if thevoltage over the DC-link tends to decrease, the grid-inverter takespower from the grid.

If the voltage over the DC-link is the same as the rated value (800V_(DC)) and the wind-turbine rotates within its pre-defined speed range,the rotor-inverter is started; that means that the control module 80starts to operate the PWM of the rotor-inverter, triggering and nottriggering, respectively, each IGBT 78 of the halfbridges (71-73) of therotor-inverter. With the resulting rotor-current/rotor-voltage, thecontrol module 80 controls the stator-side (as the generator acts as atransformer). On the stator-side, the control module 80 measures an ACvoltage (Us in the drawings, sometimes also known as U_(SYNC)) andcontrols the rotor-inverter (adjusting the PWM) until this statorvoltage Us is identical with the grid-voltage U_(G). Once both voltagesare identical, the switch 6 is closed, thus connecting the statorwindings to the grid. With the PWM of the rotor-inverter it is nowpossible to control the active and reactive power of the total powersupplied to the grid.

The power-electronic components of the converter 7 need to be protectedagainst high currents (over-currents) and over-voltages that can appearin the control lines 8 connecting the rotor windings with therotor-inverter. For example, if there is a short-circuit in the grid102, the generator 2 feed high stator-currents (I_(S)) into theshort-circuit and the rotor-currents increase very rapidly. In order toprotect the generator and the converter, the switch 6 connecting thegenerator to the grid is then opened, but there is a substantial delay(typically around 50 ms) before disconnection actually takes place, andduring this time, the high rotor-currents can harm the converter.

In order to protect the converter, it is known to provide the converter7 with a so-called “crowbar” 90, arranged so as to short-circuit therotor windings, when necessary, so as to absorb the rotor-currents andprevent them from entering the rotor-inverter and harming componentsthereof. A typical example of the basic layout of a known crowbar isshown in FIG. 2. Basically, the crowbar comprises three branchesarranged in parallel, each branch comprising two diodes (91, 92; 93, 94;95, 96) connected in series. Between the two diodes in each branch,there is a point of connection of the crowbar to the respective rotorwinding. In series with the three branches comprising the diodes, thereis a further branch comprising a power thyristor 98 and, optionally, aresistor 97. The crowbar is operated in the following manner:

In normal operation, the thyristor 98 is blocked, so that no-currentflows through the thyristor. Thus, no currents can flow through thediodes 91-96, and the rotor-currents are all fed to the rotor-inverter(71-73) of the converter 7, through the control lines 8. Now, when thereis a large increase in the rotor-currents, these currents overload theIGBTs of the rotorinverter and the PWM of the IGBTs 78 will be stopped(that is, the operation of the IGBTs is stopped) by the control module80 (the control module 80 reads the value of the current I_(L) throughthe control lines 8 and is programmed to stop operation of the IGBTswhen said currents rise above a certain level). The rotor-currents willthen flow through the free-wheel diodes 79, causing the voltage over theDC-link 77 to increase. This increase Is detected by the control module80, and once the voltage over the DC-link reaches a predeterminedthreshold, the control module fires the power thyristor 98 of thecrowbar, permitting the currents to flow through said thyristor. Then,the high rotor-currents will start to flow through the diodes of thecrowbar instead of through the rotor-inverter. The rotor-voltage will benearly zero, as the crowbar acts as a shortcircuit.

Next, the switch 6 is opened, thus disconnecting the stator 2 from thegrid; the generator will then be demagnetised over said switch 6 and thecrowbar 90. After this, the generator can be connected to the gridagain, once the grid-voltage has returned to the rated value.

FIGS. 3A-3G show, using the same time axis, the development of some ofthe parameters of a system according to FIG. 1 with a prior art crowbaras per FIG. 2, when a short-circuit is produced in the grid. Thefollowing points of time are referred to:

t1: time when the short-circuit occurs in the grid

t2: time when the crowbar is triggered

t3: time when the generator is disconnected from the grid (by openingswitch 6)

t4: time when the generator is reconnected to the grid (by closingswitch 6)

FIG. 3A shows the drop of U_(G) by the time t1 (time for short circuitin the grid).

FIG. 3B shows the stator-current I_(S). At t1, the stator-current startsto increase rapidly and it remains on a high level until the time t3,when the switch 6 is opened, thus disconnecting the stator from the grid(then, the stator-current is interrupted). Later, once the voltage onthe grid has returned to its rated value, the generator is reconnectedto the grid (at t4) and the stator-currents start to flow again.

FIG. 3C shows how the rotor-current I_(R) changes almost in the same wayas the stator-current (due to the fact that the rotor and stator act asthe primary and secondary sides of a transformer). The only differenceis due to the fact that the magnetising current for the generator iscoming from the rotor-side. Thus, in FIG. 3C, shortly before t4, a smallmagnetising current can be observed.

FIG. 3D shows the current from the rotor to the rotor-inverter (I_(L)).At t1, this rotor-inverter current increases rapidly (following theincrease in the rotor-currents, which are all fed to therotor-inverter). The rotor-inverter is stopped by the control module 80and the current then flows through the free-wheel diodes 79, into theDC-link. The voltage over the DC-link (U_(DC)) (cf. FIG. 3E) increasesvery fast, until it reaches a certain level. Then, by the time t2, thecrowbar is triggered by the control module (which has been reading thevoltage over the DC-link). The rotor-current is then commutated into thecrowbar (and I_(L) almost immediately sinks to zero, that is, no currentis fed from the rotor into the converter 7). Once the voltage is back onthe grid, the rotor-inverter starts to supply the magnetising current tothe rotor of the generator, and synchronises with the grid. Afterconnection of the generator to the grid )at t4), the rotor-currentincreases again to the rated value (cf. FIG. 3C) (if there is enoughenergy in the wind).

In FIG. 3E, it is shown how, at t1, the DC-link is charged rapidly (thevoltage U_(DC) over the DC-link thus increases). At t2, the crowbar istriggered and the charging is stopped. The discharging of the DC-link isdone by the grid-inverter. The grid-inverter discharges the DC-link downto the rated value (800 V_(DC)).

FIG. 3F shows the current through the crowbar I_(CR). By the time t2,the crowbar overtakes the total rotor-current.

Finally, FIG. 3G shows the rotor-voltage U_(R). At the beginning, therotor-voltage is at its normal operation level. At t1, therotor-inverter is stopped and rectified rotor-voltage jumps to the levelof the DC-link. The rotor-voltage increases with the voltage over theDC-link, until t2, when the crowbar is triggered; then, the rotor isshort-circuited and the rotor-voltage sinks to zero. Once the switch 6is opened and the generator is disconnected from the grid, the crowbaris opened again. Once the grid-voltage is back at its rated value again,the rotor-inverter is synchronised and the rotor-voltage is back at itsnormal operation level again.

The disconnection of the generator from the grid, as in the aboveexample, has traditionally been used so as to protect the generator andconverter when problems occur on the grid (such as short-circuits givingrise to rotor-current surges), and also for reasons related with thenetwork management. Traditionally, the disconnection has not beenconsidered to imply any substantial problems in what regards theover-all supply of power to the grid, as the wind-power generators haverepresented a very small part of the total power supplied to the grid(typically, below 5% of the total power supply). However, in manycountries, wind-power generation is representing a rapidly increasingportion of the electric power generation; and in some countries thewind-power generation represents such an important part of the totalpower generation that sudden disconnection of the wind-power generatorscan cause severe problems to the over-all electric power distributionover the grid.

Thus, it is desired to provide an arrangement that can operateappropriately without the need for disconnecting the generator from thegrid in the case of a short-circuit in the grid.

However, in the prior art arrangement described above and using thecrowbar 90 for protecting the converter 7, it is necessary to disconnectthe generator from the grid, as the triggered crowbar creates a hardshort-circuit on the rotor side. If the stator were not disconnectedfrom the grid, this short-circuit -of the rotor would produce a stableover-current in the rotor and stator windings. The rotor-voltage duringnormal operation is, with rated grid voltage-and-slip, around 200 Vrms.If the rotor is short-circuited and if the stator is not disconnectedfrom the grid, during a long time there will be over-currents in theorder of, typically, three times the rated current. If the crowbar thenis disconnected, these over-currents will “jump” into the rotor-inverterand produce an over-voltage on the DC-link 77. Then, the crowbar 90 willbe triggered again, etc. Basically, there is no way of getting out ofthis loop. Thus, In order to avoid these long-time over-currents, thestator must be disconnected from the grid.

In the above-mentioned JP-A-07-067393 and JP-A-07-194196, the probleminvolved with the voltage drop in the grid is solved by means of addinga chopper circuit in parallel with the DC-link. The rotor-currents thenflow through the free-wheel diodes of the rotor-inverter and charge theDC-link. When the voltage over the DC-link rises above a pre-determinedlevel, a chopper in series with a resistor is activated and the voltageover the DC-link is limited by discharging the DC-link over the choppercircuit. However, this solution requires that the free-wheel diodes ofthe rotor-inverter are chosen so as to support high currents (as therotor currents will continue to flow through the free-wheel diodes ofthe rotor-inverter). Further, the chopper needs a switch that can beswitched off, like a GTO or IGBT, that is, an active switch. Further,for protection reasons, there must be a crowbar arranged in parallelwith the rotor-inverter.

It is an object of the present invention to provide an arrangement thatprovides for protection of the converter without any need fordisconnecting the stator from the grid in the case of a short-circuit inthe grid, and which does not require any oversizing of the free-wheeldiodes and, preferably, no active switch. Preferably, the arrangementshould not require any crowbar.

DESCRIPTION OF THE INVENTION

A first aspect of the invention relates to a control system for adouble-fed induction generator (DFIG) comprising a rotor having rotorwindings and a stator having stator windings connectable to a grid forelectric power distribution, said control system comprising a converter,said converter comprising the following components:

a rotor-inverter connectable to the rotor windings of the generator,

a grid-inverter connectable to the grid and/or to the stator windings,and

a DC-link for feeding the rotor-inverter.

According to the invention, the converter further comprises a clampingunit for protecting the converter from damage due to over-currents inthe rotor windings, said clamping unit being connectable over the rotorwindings and arranged to be triggered from a non-operating state to anoperating state following detection of an over-current in therotor-windings, said clamping unit comprising a clamping elementarranged so that when the clamping unit is in its non-operating state,currents in the rotor windings cannot pass through said clampingelement, and when the clamping unit is in its operating state, currentsin the rotor windings can pass through said clamping element. Theclamping element comprises at least one passive voltage-dependentresistor element for providing a clamping voltage over the rotorwindings.

The voltage-dependent resistor element can be chosen so that, for anyexpected value of the rotor-currents occurring during short-circuit inthe grid, an appropriate clamping voltage will be obtained over theclamping element and, thus, over the rotor windings. It is importantthat said clamping voltage be within a predetermined range. Especially,it should not be allowed to be too low, as a too low clamping voltagewould imply that the currents in the rotor windings would decrease veryslowly (as long as the stator remains connected to the grid). Actually,if the clamping voltage is below the level of the rotor voltage duringnormal operation, the rotor currents will never go down to zero.

It is desired that the rotor-currents decrease as rapidly as possible,so as to allow the converter to start to operate again, by means ofbringing the clamping unit back to its non-operating state (whereby therotor-currents are commutated to the rotor-inverter again), so that theconverter can take over the control of the generator again. It isconsidered to be important that the converter will be able to take overthe control of the rotor currents as soon as possible, so as to be ableto control the power output to the grid also during the duration of theshort-circuit on the grid (this is normally required by the operator ofthe grid).

It is thus important that the clamping element be a voltage-dependentresistor element, so that the voltage will not be a purely linearfunction of the rotor currents: the use of a normal resistor would implythat the clamping voltage would be (substantially) directly proportionalto the rotor currents at each moment. Were a resistor chosen, care wouldhave to be taken so as to choose a resistance value low enough to makesure that the clamping voltage would never exceed a maximum levelallowed for the rotor-voltage, not even if the current flowing throughthe resistor would reach the highest level of rotor-current that couldbe expected. However, such a low value of the resistance might give riseto a too low level of the clamping voltage if the actual rotor-currentsproduced due to a short-circuit in the grid would be of a level muchlower than said highest level that could be expected. In such a case,with a too low clamping voltage, the rotor-currents would not decreaserapidly enough so as to allow the converter to take over the controlagain, or at least not in order to take over the control as rapidly asone might desire. The use of a low resistance resistor would cause ahigh steady-state over-current in the rotor windings, at ratedrotor-voltage.

However, using a voltage-dependent resistor element, it is possible tochoose this element so as to provide a rather well-defined clampingvoltage, within a rather short range, for a large range of possiblerotor-currents. Actually, there are elements that can provide for asubstantially constant clamping voltage for any value of the level ofthe rotor-current, within a very large range, basically including thefull range of possible rotor-current levels that could be expected tooccur due to a short-circuit in the grid.

Using a passive voltage-dependent resistor element is especiallyadvantageous, as it provides for a rather well-defined clamping voltagewithout requiring any complex control of the clamping unit. Basically,it is enough to trigger the clamping unit so as to allow therotor-currents to pass through the clamping unit instead of through therotor-inverter. For triggering the clamping unit, a simple triggerelement such as a power thyristor can be used, which can be arranged inseries with the clamping element(s) and the respective rotor winding andbe triggered from the control module using a very low current (forexample, below 1 A, applied through a simple pulse-transformer). Theclamping of the voltage over the rotor-windings is achieved by thevoltage-dependent resistor element itself, and no further control isneeded. That is, no “active” control of this clamping voltage is needed;once the stator-current is below its rated value, the control module cansimply stop the triggering of the thyristors and, thus, stop therotor-currents from flowing through the clamping unit after the nextzero-crossing of the current through the thyristor.

The clamping element can comprise a plurality of passivevoltage-dependent resistor elements, arranged in parallel, therebyallowing very high rotor-currents to flow through the clamping elementwithout harming the individual passive voltage-dependent resistorelements.

The passive voltage-dependent resistor element(s) can (each) comprise:

a varistor (or a plurality of varistors, connected in series);

a zener diode (or a plurality of zener diodes, connected in series);and/or

a suppression diode (or a plurality of suppression diodes, connected inseries).

Examples of suitable passive voltage-dependent resistor elements are thefollowing ones:

varistor: B80K320 from the manufacturer EPCOS;

suppression diode: BZW50-180 from the manufacturer ST

zener diode: BZG05C100 from the manufacturer Vishay

The clamping unit can comprise, for each phase of the rotor, a connectorfor connection to the respective rotor phase, each connector beingconnected to a trigger branch comprising, in series: a point ofconnection of the clamping unit to the connector for connection to therespective rotor phase; a thyristor for triggering the clamping unit;the clamping element; a diode; and the point of connection to theconnector for connection to the respective rotor phase. The clampingunit can further comprise a resistor coupled in parallel with theclamping element.

The clamping unit can be arranged to be triggered from a non-operatingstate to an operating state:

when the voltage over the DC-link rises above a pre-determined level(that is, the over-current in the rotor windings is detected bymeasuring the voltage over the DC-link);

when the voltage over the rotor-windings rises above a pre-determinedlevel (that is, the over-current in the rotor windings is detected bymeasuring the voltage over the rotor-windings).

when the currents in the rotor-windings rise above a pre-determinedlevel (that is, the over-current in the rotor windings is detected bymeasuring the currents in the rotor-windings); and/or

when the currents in the stator-windings rise above a pre-determinedlevel (that is, the over-current in the rotor windings is detected bymeasuring the currents in the stator-windings).

A second aspect of the invention relates to a double-fed inductiongenerator (DFIG) system comprising a rotor having rotor windings and astator having stator windings connectable to a grid for electric powerdistribution, said double-fed induction generator system furthercomprising a control system as described above, the rotor inverter beingconnected to the rotor windings of the generator, the grid inverterbeing connected to the grid, and the clamping unit being connected overthe rotor windings.

A third aspect of the invention relates to a method for protecting theconverter in a power generation system comprising a double-fed inductiongenerator (DFIG) comprising a rotor having rotor windings, a statorhaving stator windings connected to a grid for electricpower-distribution and a control system comprising a converter, saidconverter comprising a rotor-inverter connected to the rotor windings ofthe generator, a grid-inverter connected to the grid and/or to thestator windings, and a DC-link for feeding the rotor-inverter. Themethod comprises the steps of:

connecting a clamping unit having a clamping element over the rotorwindings, said clamping unit comprising a clamping element arranged sothat when the clamping unit is in its non-operating state, currents inthe rotor windings cannot pass through said clamping element, and whenthe clamping unit is in its operating state, currents in the rotorwindings can pass through said clamping element, said clamping elementcomprising at least one passive voltage-dependent resistor element forproviding a clamping voltage over the rotor windings; and

triggering the clamping unit to its operating state when an over-currentis detected in the rotor windings.

The clamping unit can be triggered from a non-operating state to anoperating state, for example,

when the voltage over the DC-link rises above a pre-determined level,

when the voltage over the rotor-windings rises above a pre-determinedlevel,

when the currents in the rotor-windings rise above a pre-determinedlevel, and/or

when the currents in the stator-windings rise above a pre-determinedlevel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a DFIG system according to the state of theart.

FIG. 2 schematically shows a crowbar according to the state of the art.

FIG. 3 schematically illustrates the changes in some of the parametersof the system according to the state of the art, during a time-periodfollowing a short-circuit in the grid.

FIG. 4 schematically illustrates the changes in some of the parametersof the system according to a preferred embodiment of the invention,during a time-period following a short-circuit in the grid.

FIG. 5 schematically shows a system according to a preferred embodimentof the invention.

FIG. 6 schematically shows a system according to another preferredembodiment of the invention.

FIG. 7 schematically shows a clamping unit according to a preferredembodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIGS. 5 and 6 illustrate two preferred embodiments of the invention.Most of the illustrated components correspond exactly to those of theprior art system described referring to FIG. 1; these components bearthe same reference numerals and need no further description. However,instead of the converter 7 of FIG. 2, FIGS. 5 and 6 illustrateconverters comprising the same basic elements but:

FIG. 5 illustrates a converter 170 where the crowbar has been replacedby a clamping unit 190, the converter comprising a control module 180adapted to control said clamping unit (apart from that, the controlmodule 180 operates as the control module 80 of the system of FIG. 1);and

FIG. 6 illustrates a converter 171 where a clamping unit 190 has beenincorporated in parallel with the crowbar 90, the converter comprising acontrol module 181 adapted to control said clamping unit and crowbar(apart from that, the control module 181 operates as the control module80 of the system of FIG. 1).

FIG. 7 illustrates a preferred embodiment of the clamping unit clampingunit comprising, for each phase of the rotor, a connector (300) forconnection to the respective rotor phase. Each connector is connected toa trigger branch comprising, in series: a point of connection (297) ofthe clamping unit to the connector (300) for connection to therespective rotor phase; a thyristor (295) for triggering the clampingunit; the clamping element (290); a diode (296); and the point ofconnection (297) to the connector (300) for connection to the respectiverotor phase.

The thyristor 295 and the diode 296 can be integrated in one singlecomponent, such as SKKH210/12E from Semikron. The clamping element 290can be a varistor such as B80K320 from EPCOS.

(Instead of the diode 296, a thyristor could be used, with the advantagethat the delay between the stopping of the triggering of the clampingunit and the actual stopping of the current flowing through the clampingunit is reduced by up to 50%, compared to when a diode is used).

FIG. 4A shows the grid-voltage, with a short-circuit appearing by t11.Then, the stator-current I_(S) (FIG. 4B) increases rapidly. However, inthis case, the generator is not disconnected and will be demagnetizedover the stator and rotor-currents and the stator- and rotor-currentswill then decrease. Once the stator-currents are below the rated level(approximately by t13) the clamping unit 190 will be opened by thecontrol module (180, 181) and the rotor currents will flow into therotor inverter again. The converter measures the rotor-currents (bymeasuring the currents I_(L) in the control lines) and synchronises thePWM with those currents. The rotor-inverter controls the rotor-currentsand provides for a constant rotor- and stator-current all through theremaining duration of the short-circuit (from t14 to t15 in FIGS. 4B and4C). Later, when the grid-voltage returns to its rated value, thegenerator is not magnetised enough and from the grid a high currentflows to the stator and produces an over-current (in the intervalbetween t15 and t17 in FIG. 4C). The rotor-inverter is then stoppedagain, and the generator will be magnetised from the grid. After this,the stator-current decreases and, once it is under the rated value(t17), the clamping unit 190 is opened and the rotor-inverter overtakesthe control of the rotor-current again.

The rotor-current (I_(R)) (FIG. 4C) is nearly the same as thestator-current.

FIG. 4D shows the current I_(L) to the rotor-inverter (that is, thecurrent from the rotor to the converter). At t11, the rotor-currentincreases rapidly and at t12, the clamping unit 190 is triggered (justas the crowbar was triggered in the prior art system described above).Thus, the rotor-current is commutated into the clamping unit 190 andI_(L) sinks to zero. Once the stator-current sinks below the ratedcurrent (t13), the clamping unit is opened by the control module (180,181) and the rotor-current is commutated into the rotor-inverter. Therotor-inverter synchronises with the rotor-current and controls thecurrent during the remaining part of the duration of the short-circuit(t14-t15). (In the time between t14 and t15, the grid operator requiresthat the wind-turbine actively supplies a current in the short circuitoutside the wind-turbine, in order to provoke a more rapid disconnectionof the short-circuit by opening a high voltage circuit breaker in thegrid. Inter alia for this purpose, the generator should be controlledduring most of the duration of the short-circuit. The invention allowsthe rotor-inverter to be stopped only in the transient timescorresponding to the presence of the dynamic over-currents caused byfast voltage changes on the grid).

When the grid-voltage returns to its rated value (t15), therotor-current increases rapidly and the rotor-inverter is stopped again(as an over-current is measured by the control module), the clampingunit 190 is triggered and overtakes the rotor-current. When thestator-current sinks below the rated level (t17), the clamping unit isopened and the rotor-current is commutated into the rotor-inverteragain. The rotor-inverter synchronises with the actual rotor-current andstarts to operate again, controlling the rotor-current.

FIG. 4E shows the voltage over the DC-link. At t11, there is a firstspike, which triggers the clamping unit 190 (at t12). Later, asdescribed above, the clamping unit is opened and the rotor-current iscommutated into the rotor-inverter again, starting to charge the DC-linkagain (t13), until the rotor-inverter overtakes the control of therotor-current (this is by the control module—180, 181—, so as toovertake the control of the generator again). This happens two times inFIG. 4E, first due to the voltage drop on the grid and the second timewhen the grid-voltage rises again.

FIG. 4F shows the clamping current (current through the clamping unit)I_(CL). The clamping unit overtakes the full rotor-current two times, asoutlined above.

FIG. 4G shows the rotor-voltage U_(R).

Initially, the rotor-voltage is at its normal operation level. By t1,the rotor-current increases and the rotor-inverter is stopped. Therotor-current is like a current source and flows over the free-wheeldiodes 79, into the DC-link 77. Here, the rotor-voltage will be at thesame level as the voltage over the DC-link.

The rotor-voltage increases with the increasing DC-link voltage and byt12, the clamping unit is triggered and the rotor-voltage is clamped toa level in accordance with the chosen characteristics of the clampingelement 290. By t13 the clamping unit is opened and the rotor-currentflows into the rotor-inverter and the rotor-voltage jumps to the levelof the DC-link voltage. After a time for synchronising therotor-inverter with the actual rotor-current, the rotor-inverter startsto work (t14) and the level of the rotor-voltage is returning to thelevel corresponding to normal operation. While the short-circuitcondition remains on the grid (t14-15), the “normal” rotor-voltage islower then before t1, because of the drop of the stator-voltage.

When the voltage returns to the grid (t15), the system will react as bythe voltage drop:

by t15, the rotor-current rises fast and the rotor-inverter is stopped;the rotor-voltage increases to the level of the DC-link voltage;

by t16, the clamping unit is triggered, and the rotor-voltage is clampedto a level defined by the characteristics of the clamping element 290;

by t17, the clamping unit is opened and the rotor-current flows into therotor-inverter, and the rotor-voltage jumps to the level of the DC-linkvoltage;

finally, after the time for synchronising the rotor-inverter with theactual rotor-current, by t18 the rotor-inverter starts to work again)

FIG. 4H shows the clamping voltage U_(CL). Ideally, the clamping voltagewill change between two well-defined levels, namely, between zero and aclamping level.

Throughout the description and claims of the specification, the word“comprise” and variations of the word, such as “comprising”, is notintended to exclude other additives, components, integers or steps.

1. A control system for a double-fed induction generator (DFIG)comprising a rotor (1) having rotor windings and a stator (2) havingstator windings connectable to a grid for electric power distribution;said control system comprising a converter (170, 171), said convertercomprising the following components: a rotor-inverter (71-73)connectable to the rotor windings of the generator, a grid-inverter(74-76) connectable to the grid and/or to the stator windings, and aDC-link (77) for feeding the rotor-inverter; the converter (170, 171)further comprising a comprising unit (190) for protecting the converterfrom damage due to over-currents in the rotor windings, said clampingunit (190) being connectable over the rotor windings and arranged to betriggered from a non-operating state to an operating state followingdetection of an over-current in the rotor-windings, said clamping unitcomprising a clamping element (290) arranged so that when the clampingunit is in its non-operating state, currents in the rotor windingscannot pass through said clamping element, and when the clamping unit isin its operating state, currents in the rotor windings can pass throughsaid clamping element, said clamping element comprising at least onepassive voltage-dependent resistor element (291, 292, 293, 294) forproviding a clamping voltage over the rotor windings.
 2. A controlsystem according to claim 1, wherein the clamping element (290)comprises a plurality of passive voltage-dependent resistor elements(291, 292, 293, 294), arranged in parallel
 3. A control system accordingto claim 1 or 2, wherein said at least one passive voltage-dependentresistor element comprises at least one varistor.
 4. A control systemaccording to claim 1 or 2, wherein said at least one passivevoltage-dependent resistor element comprises at least one zener diode.5. A control system according to claim 1 or 2, wherein said at least onepassive voltage-dependent resistor element comprises at least onesuppression diode.
 6. A control system according to any of the precedingclaims, wherein the clamping unit comprises, for each phase of therotor, a connector (300) for connection to the respective rotor phase,each connector being connected to a trigger branch comprising, inseries: a point of connection (297) of the clamping unit to theconnector (300) for connection to the respective rotor phase; athyristor (295) for triggering the clamping unit; the clamping element(290); a diode (296); and the point of connection (297) to the connector(300) for connection to the respective rotor phase.
 7. A control systemaccording any of the preceding claims, wherein the clamping unit furthercomprises a resistor (298) coupled in parallel with the clamping element(290).
 8. A control system according to any of the preceding claims,wherein the clamping unit is arranged to be triggered from thenon-operating state to the operating state when the voltage over theDC-link rises above a pre-determined level.
 9. A control systemaccording to any of claims 1-7, wherein the clamping unit is arranged tobe triggered from the non-operating state to the operating state whenthe voltage over the rotor-windings rises above a pre-determined level.10. A control system according to any of claims 1-7, wherein theclamping unit is arranged to be triggered from the non-operating stateto the operating state when the currents in the rotor-windings riseabove a pre-determined level.
 11. A control system according to any ofclaims 1-7, wherein the clamping unit is arranged to be triggered fromthe non-operating state to the operating state when the currents in thestator-windings rise above a pre-determined level.
 12. A double-fedinduction generator (DFIG) system comprising a rotor (1) having rotorwindings and a stator (2) having stator windings connectable to a gridfor electric power distribution, said double-fed induction generatorsystem further comprising a control system according to any of thepreceding claims, wherein the rotor inverter (71-73) is connected to therotor windings of the generator, the grid inverter (74-76) is connectedto the grid, and the clamping unit (190) is connected over the rotorwindings.
 13. A method for protecting the converter in a powergeneration system comprising a double-fed induction generator (DFIG)comprising a rotor (1) having rotor windings, a stator (2) having statorwindings connected to a grid for electric power distribution and acontrol system comprising a converter (170, 171), said convertercomprising a rotor-inverter (71-73) connected to the rotor windings ofthe generator, a grid-inverter (74-76) connected to the grid and/or tothe stator windings, and a DC-link (77) for feeding the rotor-inverter;whereby the method comprises the steps of: connecting a clamping unit(190) having a clamping element over the rotor windings, said clampingunit comprising a clamping element (290) arranged so that when theclamping unit is in a non-operating state, currents in the rotorwindings cannot pass through said clamping element, and when theclamping unit is in an operating state, currents in the rotor windingscan pass through said clamping element, said clamping element comprisingat least one passive voltage-dependent resistor element (291, 292, 293,294) for providing a clamping voltage over the rotor windings; andtriggering the clamping unit from its non-operating state to itsoperating state when an over-current is detected in the rotor windings.14. A method according to claim 13, wherein the clamping unit istriggered from the non-operating state to the operating state when thevoltage over the DC-link rises above a pre-determined level.
 15. Amethod according to claim 13, wherein the clamping unit is triggeredfrom the non-operating state to the operating state when the voltageover the rotor-windings rises above a pre-determined level.
 16. A methodaccording to claim 13, wherein the clamping unit is triggered from thenon-operating state to the operating state when the currents in therotor-windings rise above a pre-determined level.
 17. A method accordingto claim 13, wherein the clamping unit is triggered from thenon-operating state to the operating state when the currents in thestator-windings rise above a pre-determined level.